This invention relates to a multi-stage hydrotreating process for heavy hydrocarbon oils. More particularly, this invention is concerned with the improvement in a process comprising first subjecting heavy hydrocarbon oils to demetallation, and then subjecting the same to hydrodesulfurization.
In recent years, there is every indication that crude oils become heavier and heavier. In the petroleum processing industry, furthermore, there is a growing tendency to subject feed oils to deep-drawing at the stages of atmospheric distillation and vacuum distillation. This comes to attach more importance to hydrotreating of heavy hydrocarbon oils.
The heavy hydrocarbon oils, whose typical examples are atmospheric distilled residuum and vacuum distilled residuum, contain large amounts of metallic contaminants such as vanadium, nickel, iron and the like as well as asphaltene and the like. Accordingly, when hydrotreating the heavy hydrocarbon oil of this sort, a hydrotreating catalyst is generally protected from poisoning caused by the metallic contaminants in the feed oil in the manner of first contacting the feed oil with a demetallation catalyst and then contacting it with a hydrotreating catalyst. As the demetallation catalyst in this case, there is normally used a catalyst comprising supporting Group VI and Group VIII metals on an alumina carrier, said metals being within the range of 8 to 20 wt.% in terms of oxide, and having a macro-pore whose diameter is 600 angstrom or more and a large average pore diameter. As the hydrotreating catalyst, in particular hydrodesulfurizing catalyst, there is generally used a catalyst that is substantially identical in the composition as compared with said demetallation catalyst but has a pore smaller than that of said demetallation, namely an average pore diameter within the range of about 90 angstrom to about 180 angstrom.
In a two-stage process comprising first passing a heavy hydrocarbon oil through a demetallation catalyst under hydrotreating conditions, and then passing it through a hydrotreating catalyst bed, the amounts of coke and metal deposited on said demetallation catalyst bed increase with the lapse of treating hours. Owing to this, the activity of the demetallation catalyst lowers, and the pressure drop on the demetallation catalyst bed increases too. In this situation, there is no way but interrupting the hydrotreating operation, even if a hydrotreating catalyst bed on the understream side is still maintaining a sufficient activity. Accordingly, as the amounts of metallic contaminants contained in the heavy hydrocarbon oil, namely feed oil, increase, it is unavoidable to frequently interrupt the running of the process and exchange the catalyst used. Further, the two-stage process is troublesome in that when the demetallation catalyst bed is hardened by deposition of coke and metal, there must be employed means such as blasting to exchange the catalyst.